The Upper Tail of Flood Peaks Over China: Hydrology, Hydrometeorology, and Hydroclimatology

Characterizing the upper tail of flood peak distributions remains a challenge due to the elusive nature of extreme floods. China has experienced some of the most extreme floods on Earth and is therefore an important setting for extreme flood research. Here, we examine the upper tail of flood peaks over China based on a comprehensive flood dataset that integrates systematic observations from 1759 stream gaging stations and 14,779 historical flood surveys. Floods with large upper tail ratios (i.e., flood peak discharge normalized by sample 10-yr flood) cluster spatially in the windward regions of mountainous terrain with basin mean annual rainfall of around 550 mm. Large upper tail ratio floods are associated with a mixture of rainfall generation processes. These processes are characterized by anomalous moisture transport (e.g., zonal water vapor pathways) and/or synoptic configurations (e.g., blocking), with respect to flood-producing storms for annual flood peaks. Downscaling simulations with the Weather Research and Forecasting model for the 1939, 1963, and 2016 floods over the North China Plain region highlight the role of interactions between complex terrain and large-scale environment in defining the world envelope curve of flood peaks. The upper-tail floods demonstrate a combination of ingredients producing extreme rainfall. Our results provide important insights to extreme floods in China and a frame of reference for assessing the largest floods on Earth.